It was between the years 1879, when the society began, and 1944, that they officially taught the destruction of Jerusalem in 606 BC
The last official mention of that date found in WT literature is in the WT magazine of 1st July 1942.
There was a hiatus of two years when no further mention was made.
Then with the publication of "The Kingdom is at Hand", in 1944, the date was officially changed to 607 BC [see pgs 168,169, 176, 183]
The present publication being studied at the book studies "Revelation Its Grand Climax", makes a half-true statement on pg 105, when it asserts, wih apparent confidence, that this adjustment was made a year earlier in the book The Truth Shall Set You Free" This is only partly true because that same publication split the dates for Jerusalem's destruction in 607, and the start of the Gentile Times in 606 BC, as two seperate dates.
It was only in the next year with the "KIngdom is at Hand" book that the two dates were finally reconciled into the current dating system.
Oddly enough, the "Revelation Climax" puts this adjustment down, not to divine intervention, but to "providence" Come now. Do they believe, like the "pagans" believe, in "providence" ?
Summary: 1879 - 1944.................................. 606 BC.......................... 65 years
1944 - today..................................607 BC..........................63 years
So they have been teaching error for a longer period than they have been teaching "truth" !!!
This same "providence" that the WTS apparently relies on, would have disfellowshipped the first two Presidents of the WTS for teaching error.
Hope this helps
Cheers